Software Defined Radio Use Case for Spectrum Monitoring

Software Defined Radios (SDRs) are highly functional and advanced radio communication devices that are employed in numerous applications across various industries. The reason behind their ubiquitousness is their line-up of beneficial characteristics that include high functionality, ruggedness, versatility, compactness, wide compatibility, and ease of integration. This article is the third in a series that discusses the use cases and applications of SDRs. Previous articles have covered SDR applications in GPS/GNSS and radar, and future articles will discuss their uses in defense, test & measurement, interoperability, and satellite applications.

Today's focus is the use of SDRs in spectrum monitoring. This covers a wide range of applications such as testing new cellular networks, protecting aviation channels, monitoring restricted areas for illegal broadcasts, and protecting military assets from RF attacks.

A high-performance spectrum monitoring solution is vital for enforcing regulations in the radio spectrum and ensuring optimal performance in networks. As the spectrum becomes progressively congested, solutions such as SDR for spectrum monitoring are becoming a de facto standard for ensuring that RF systems are operating as they should.

What is spectrum monitoring? Why do we need it?

As the name implies, spectrum monitoring is a spectrum management process that involves monitoring a portion of the radio frequency spectrum and recording the captured data for analysis. The process of spectrum monitoring involves three major steps: monitoring, recording, and playback & analysis.

During monitoring, a receiver listens for and receives signals within a selected spectrum. The next step is recording, in which the vast amounts of data captured during monitoring are streamed (as raw IQ data when using SDRs) to what is usually a server system storage solution, often using Ethernet protocols. Note that other systems of storage and data transfer exist, but the aforementioned are the most widely used.

Recording is followed by playback and analysis. Using dedicated spectrum monitoring software, digital signal processors (DSPs) playback the data, hone into the signal of interest, and perform analysis and characterization on the signals captured. This could be for signal detection and identification, geolocation and mapping, demodulation for listening, etc.

A spectrum monitoring system comprises of a spectrum analyzer. Realtime spectrum signal analyzers (RTSA), which are devices that are capable of receiving, processing, and displaying signals in real time, are usually preferred in spectrum monitoring systems. These systems may also require the use of a storage solution and a user interface for control and visualization.

Often, spectrum monitoring is the first step in preventing signal attacks such as signal jamming, interference, intrusion, etc, as it detects and identifies these problems. Spectrum monitoring can also be used to determine spectrum availability, characterize RF signals and emissions, analyze transmission patterns, continually monitor critical frequency bands to ensure they’re clear, detect illicit transmissions, and manage assigned or regulated spectrum.

Other reasons why we need spectrum monitoring are detecting and locating illicit surveillance devices in government facilities, enterprises, and other organizations; conducting 24/7 and wireless continuous facility monitoring from a central location; and protecting sensitive information and intellectual property from malicious actors.

Applications

Spectrum monitoring performs various functions and has numerous uses. These functions are applied in several industries to create specific, industry-related applications.

One of these is testing cellular networks. The advent of 5G ultra-low latency and high-bandwidth communications has brought about numerous competing users in various applications such as drone delivery services, IoT devices for the IIoT (Industry 4.0), and self-driving cars. Spectrum monitoring is used extensively for testing networks in different applications.

5G new radio (NR) transmissions are time division duplexed (TDD) signals. As a result, the transceivers in mobile phones continually switch between synchronization signal block (SSB), downlink, and uplink. The required switches take place rapidly because the 5G NR frame is just 10 ms. This means that the test instrument utilized must be capable of capturing signals without gaps. This is one of the reasons why RTSAs are most suitable for 5G network installation.

An RTSA captures the whole signal chunk, processing it concurrently and eliminating coverage gaps inherent in swept instruments that create the possibility that the data or SSB will be missed. For the most part, 5G testing, especially power measurements, is done over the air.

Spectrum monitoring also sees wide usage at secure facilities such as airports. The FAA and the Air Force are the two top spectrum users in the US Federal Government. Frequency bands in aviation are typically allocated for three major functions: communications, navigation and surveillance, and air traffic management.

Aviation communications within and around busy metropolitan airports are particularly vital. This is because the timing and coordination of air traffic patterns and runway activity are very tight, leaving little margin for error. Monitoring for interference in adjacent channels/bands is vital to determine if swift action is required.

Frequency bands need to be monitored to guarantee that there is a specific bandwidth between the spacing of channels. This is to ensure that there is undisrupted transmission.

Certain frequencies are assigned for emergency functions; for example, 121.5 MHz is the International Aeronautical Emergency Frequency. Different services act as a fail-safe; for example, in the case of GPS navigation failure, VOR (very high frequency omnidirectional range) communication or another communications, navigation or surveillance band may be used. All these frequencies and bands have to be kept operational and free through spectrum monitoring.

Spectrum monitoring is also used to manage and enforce licensed spectrum allocation and sharing. Frequencies are assigned to governments, private entities, and organizations by the International Telecommunication Union (ITU) through regional bodies. Authorized use is ensured through regular monitoring.

Furthermore, as more and more frequencies are being assigned, it is becoming increasingly difficult to completely clear new frequency bands for future RF usage. When clearing a band is not possible, spectrum sharing may provide a way out by facilitating mobile access to additional bands in certain geographical areas and at periods when the bands are not in use by other services. Shared Spectrum Access for Similar Technologies (SSA-TA) is also monitored by the ITU through regional bodies, using spectrum monitoring.

SDRs for Spectrum Monitoring

Not just any RF device, storage device, or host system can function as part of a spectrum monitoring system. There are certain parameters that they must meet. Examples include frequency range, coverage, and span. A spectrum contains different bands, which, in turn, contain various frequencies. Spectrum monitoring requires the receiver to be able to capture signals over a wide range of frequencies. A spectrum analyzer typically has a start and a stop frequency (top and bottom of its frequency range), with the difference between them being its scan. The frequency in the center of the scan is known as the center frequency.

Another vital parameter for spectrum analyzers is phase noise/ DANL (Displayed Average Noise Level). As the name implies, this is the average noise level displayed by the analyzer. It is the measure of the sensitivity of the device and is directly proportional to frequency. A lower DANL is vital for detecting low-level signals and can be achieved using a pre-amplifier and selecting a narrow RBW.

This brings us to yet another important specification for spectrum analyzers, the resolution bandwidth (RBW). This is a measure of how close two frequency components (signals) can be and still be recognized and resolved as two separate entities. A narrow RBW results in a decreased DANL, enabling the analyzer to detect low-level signals. However, this setting is not suitable for modulating signals where sufficient RBW width is required to capture the sidebands of these types of signals. Modern digital signal processing solutions ease or eliminate the need for this trade-off by offering the best of both settings.

Crucial specifications for data transfer and storage solutions include data rate, storage space, and the efficiency of data transfer (without dropped packets). Spectrum monitoring involves large amounts of data, and fast and efficient data transfer is required. The storage solution also has to be sufficiently large.

Similarly, digital signal processing on the spectrum analyzer has to be fast, functional, versatile, and efficient.

Due to these many parameters needing to be met, Software Defined Radios are a great solution for spectrum monitoring. The inherent characteristics of the technology meet and surpass the requirements and specifications of spectrum monitoring, recording, and analysis. Before going into the details on why this is the case, a good understanding of what SDRs are is required.

As the name implies, an SDR is a radio device that performs much of the function typically performed by hardware, using software instead. SDRs comprise a radio front end (RFE) and digital backend.

The RFE contains the receive (Rx) and transmit (Tx) functions and, depending on the capability of the unit, receive/transmit signals over a 0-18 GHz bandwidth. This can be upgraded to 40 GHz in Per Vices' Cyan, the highest- bandwidth SDR in existence, pictured in figure 1. An SDR may contain multiple independent Tx and Rx channels with dedicated DACs/ADCs.

An SDR’s digital backend contains an FPGA with on-board DSP capabilities for modulation/demodulation, upconversion/downconversion, signal processing etc. The FPGA is also responsible for the SDR’s configurability and upgradeability for the different radio protocols, DSP algorithms, etc.

In spectrum monitoring applications, SDRs are usually paired with a high-performance storage solution.

Figure 1: Per Vices’ Cyan is an example of an SDR.

Spectrum monitoring requires high levels of capabilities from SDR storage solutions. Spectrum recording typically involves passing a large amount of data to a host system. This operation is complex, as there are often a lot of packet dropping/loss issues, and requires a very good network interface card (NIC) (10G, 40G, 100G, PCIe, etc.) A diagram is shown in figure 2.

Per Vices use FPGA-based NICs to prevent packet dropping and nVME SSDs to support the very high data throughput required. Among the line-up of products is the highest throughput SDR - Cyan, capable of up to 400 Gbps. It is recommended to use RAID configuration to further ensure optimal writing/reading of data in spectrum monitoring.

Figure 2: This block diagram shows an SDR with a storage solution.

SDRs are wideband and can operate across or scan/sweep the spectrum across all of the frequencies that need to be kept clear and/or be used as an RTSA over a wide bandwidth. They can be used for cognitive radio applications needed for spectrum sharing research and development. The best performance spectrum monitoring receivers can capture signals over a very wide range of frequencies (up to 0-40GHz).

Furthermore, SDRs can detect power levels to ensure nothing is operating outside maximum transmission power.

Benefits of SDR for spectrum monitoring

SDRs can have multiple channels and tune to different frequencies with wide bandwidth to facilitate the monitoring of all frequencies in real time. The highest channel count SDR in our lineup has 16 independent radio chains.

SDRs can be used in older/existing legacy RF systems due to their interoperability and ease of integration. These flexible platforms can be used not only for new deployments but also for service life extension programs (SLEP) of older air traffic control or mobile network base stations. SDRs are also upgradable and reconfigurable for different types of spectrum monitoring functions.

In addition to these, SDRs have a good SFDR and noise figure, which is important for detective unwanted, interfering, or jamming signals.

In Per Vices' products, we use an FPGA combined with the VITA49 standard, which supports high-precision timestamping of the packets with timing correction that compensates for the RF front-end delays resulting in more accurate metadata for knowledge of the exact timing of the signals. If required, on-board FPGA DSP resources can perform processing operations on incoming data such as compression, filtering, taking FFT, power measurements, etc.

Conclusion

Spectrum monitoring is a challenging and sometimes tedious process due to the vast amount of data received, processed and captured on a storage device. However, the high functionality of SDR combined with exceptional performance are especially beneficial in spectrum monitoring applications.

Click here to browse Per Vices SDR products listed on everything RF.

This is the third article in a series covering SDR applications. The previous article was Software Defined Radio Use Case for Radars.